In recent years, with improvements in the performance of physical devices (also called “physical machines” below), research has been advancing in virtualization technology wherein a plurality of virtual devices (also called “virtual machines” below) are integrated into one physical machine. This virtualization technology makes it possible, for example, to assign a physical machine to a plurality of virtual machines by virtualization software (hypervisor) and to provide services by application programs (also called simply “applications” below) which are installed in the respective virtual machines.
In a virtual machine that is operating, all or a portion of the contents (called “disk image” below) making up the virtual machine may be transferred to a virtual machine created in another physical machine. By this means, an operator providing services (simply called “operator” below) can, for example, transfer the disk image of a virtual machine created in a physical machine which is close to a 100% CPU use rate, to a virtual machine created in another physical machine which has spare capacity (see, for example, Japanese Laid-open Patent Publication No. 2000-132403 and Japanese Laid-open Patent Publication No. 2004-158012).